Crystalline organic pigment sensitizers for photoconductive layers

Abstract

A specific class of crystalline organic pigments having the formula ##STR1## WHEREIN N IS 1 OR 2; X consists of the atoms necessary to complete a naphthalene, anthracene, or ##STR2## R.sup.1, R.sup.2, and R.sup.3 are independently selected from the group consisting of hydrogen, halogen, alkoxy, NO.sub.2, alkyl, SO.sub.3 H or alkali metal salts thereof, and COOH or alkali metal salts thereof; and wherein R.sup.2 and R.sup.3 can comprise the atoms necessary to complete a phenyl, naphthyl or anthryl ring; and R is selected from the group consisting of ##STR3## or COOM wherein M is alkyl or an alkali or alkaline earth metal is useful as sensitizers for photoconductive compositions in electrophotographic processes.

Description

The process of xerography, as disclosed by Carlson in U.S. Pat No. 2,297,691, employs an electrophotographic element comprising a support material bearing a coating of a normally insulating material whose electrical resistance varies with the amount of incident actinic radiation it receives during an imagewise exposure. The element, commonly termed a photoconductive element, is first given a uniform surface adaptation. It is then exposed to a pattern of actinic radiation which has the effect of differentially reducing the potential of the surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge or electrostatic latent image remaining on the electrophotographic element is then made visible by contacting the surface with a suitable electroscopic marking material. Such marking material or toner, whether contained in an insulating liquid or on a dry carrier, can be deposited on the exposed surface in accordance with either the charge pattern or the absence of charge pattern as desired. The deposited marking material may then be either permanently fixed to the surface of the sensitive element by known means such as heat, pressure, solvent vapor, or the like, or transferred to a second element to which it may similarly be fixed. Likewise, the electrostatic latent image can be transferred to a second element and developed there.

Various photoconductive insulating materials have been employed in the manufacture of electrophotographic elements. For example, vapors of selenium and vapors of selenium alloys deposited on a suitable support and particles of photoconductive zinc oxide held in a resinous, film-forming binder have found wide application in present-day document copying applications.

Since the introduction of electrophotography, a great many organic compounds have also been screened for their photoconductive properties. As a result, a very large number of organic compounds are known to possess some degree of photoconductivity. Many organic compounds have revealed a useful level of photoconduction and have been incorporated into photoconductive compositions. Optically clear organic photoconductor containing elements having desirable electrophotographic properties can be especially useful in electrophotography. Such electrophotographic elements may be exposed through a transparent base if desired, thereby providing unusual flexibility in equipment design. Such compositions, when coated as a film or layer on a suitable support also yield an element which is reusable; that is, it can be used to form subsequent images after residual toner from prior images has been removed by transfer and/or cleaning.

Although some of the photoconductors comprising the materials described are inherently light-sensitive, their degree of sensitivity is usually low in the short wavelength portion of the spectrum so that it is common practice to add materials to increase the speed and to shift the sensitivity toward the longer wavelength portion of the visible spectrum. Increasing the speed and shifting the sensitivity of such systems into the visible region of the spectrum has several advantages: it makes available inexpensive and convenient light sources such as incandescent lamps; it reduces exposure time; it makes possible the recording of a wide range of colors in proper tonal relationship, and allows projection printing through various optical systems. By increasing the speed through the use of sensitizers, photoconductors which would otherwise have been unsatisfactory are useful in processes where high speeds are required such as document copying.

Various pigment particles are described in the prior art as useful photoconductors. The use of substituted 2,4-diaminotriazines as electrically photosensitive particles in photoelectrophoretic imaging and in conventional xerographic processes is described in U.S. Pat. No. 3,445,227. U.S. Pat. Nos. 3,447,922 and 3,448,028 describe the use of N-substituted-3,13-dioxodinaphtho(2,1-b',2',3'-d)-furan-6-carboxamides as photosensitive particles in xerographic processes. These photoconductors in themselves are not sensitive at the longer wavelength portion of the visible spectrum.

A large number of acidic compounds have been described as sensitizers for photoconductive compositions such as in U.S. Pat. No. 3,316,087 and French Pat. No. 1,288,392. The search for materials which enhance the sensitization of a broad range of photoconductors is a continuing one.

It is, therefore, an object of this invention to provide novel sensitized photoconductive compositions.

It is another object of this invention to provide novel sensitized photoconductive elements.

It is a further objective of this invention to provide a process for using novel sensitized photoconductive elements.

These and other objects are accomplished with photoconductive compositions containing a photoconductor and a sensitizing amount of a specific class of crystalline organic pigments. Electrically insulating polymeric binders are generally used with the inorganic photoconductors.

The organic pigment sensitizers which can be extracted for form the crystalline pigments of this invention are described in copending U.S. Patent application Ser. No. 466658, filed May 3, 1974, by Contois et al and have the formula: ##STR4## wherein: n is 1 or 2;

X consists of the atoms necessary to complete a naphthalene, anthracene or ##STR5##

R.sup.1, R.sup.2, and R.sup.3 are independently selected from the group consisting of hydrogen, halogen, such as chlorine, bromine, fluorine or iodine, alkoxy, preferably containing 1 to 8 carbon atoms such as methoxy, ethoxy, propoxy, and the like, including substituted alkoxy such as aminoalkoxy, haloalkoxy, and the like, NO.sub.2, alkyl, preferably containing from 1 to 8 carbon atoms such as methyl, ethyl, propyl, butyl, isopentyl, hexyl and the like, SO.sub.3 H or alkali metal salts thereof such as SO.sub.3 Na, SO.sub.3 K, and the like, and COOH or alkali metal salts thereof such as COONa, COOK, and the like.

R.sup.2 and R.sup.3 can, taken together, also comprise the atoms necessary to complete a phenyl, naphthyl, or anthryl ring.

R is selected from the group consisting of ##STR6## or COOM wherein M is alkyl preferably containing from 1 to 8 carbon atoms such as methyl, ethyl, isopropyl, butyl, hexyl, octyl, and the like; alkali metals such as Li, Na, K, Rb and Cs; and alkaline earth metals such as Ca, Sr, Ba, and the like.

It is noted that the term alkyl used throughout the specification and claims is meant to include substituted alkyl such as chloroalkyl, aralkyl, and the like. Generally, any substituent may be used on the alkyl that does not adversely affect the sensitizing properties of the pigment.

The amorphous pigments are formed into crystalline pigments by extracting the pigments in a solvent such as a chlorinated solvent like trichloroethane trichloromethane, dichloroethane, or the like. Generally, the pigments are extracted at temperatures of from about 20.degree. C to about 115.degree. C for about 2 to about 24 hours. The extraction is generally complete when the extract becomes essentially colorless.

The crystalline pigment is distinguished readily from the amorphous pigments by simply preparing an x-ray diffraction pattern by subjecting a sample of pigment in an x-ray machine and recording the x-ray scattering. The crystalline pigment shows a coherent scattering due to its crystalline order and has many sharp maximum peaks as compared to the incoherent scattering of the pattern of the amorphous pigment due to its amorphous order and which shows a broad band.

Some preferred pigment sensitizers of this invention, in crystalline form, are ##STR7## (Pigment Violet Rb manufactured by Roma Chemicals) ##STR8## (Harrison Red pigment manufactured by Harmon Colors) ##STR9## (Malta Red pigment manufactured by Hercules Company) ##STR10## (Pyrazolone Red-yellowish pigment manufactured by Harmon Colors) and ##STR11## (Diane blue pigment manufactured by Harmon Colors).

The above organic pigments produce an enhanced sensitization effect when used in a photoconductive composition containing photoconductors in comparison to the sensitization of (A) photoconductor compositions without the sensitizers, (B) photoconductive compositions containing only the pigments without an independent photoconductor, (C) photoconductive compositions containing a photoconductor and similar pigments which are outside of this invention, and (D) photoconductor compositions containing photoconductors and the same pigments in amorphous form. The enhanced sensitization is evidenced by greater electrical speeds. A suitable method of increasing electrical speeds is described more fully in the appended examples.

Preferred binders for use in preparing the present photoconductive layers comprise polymers having fairly high dielectric strength which are good electrically insulating film-forming vehicles. Materials of this type comprise styrene-butadiene copolymers; silicon resins; styrene-alkyd resins; silicon-alkyd resins; soya-alkyd resins; poly(vinyl chloride); poly(vinylidene chloride); vinylidene chloride-acrylonitrile copolymers; poly(vinyl acetate); vinyl acetate-vinyl chloride copolymers; poly(vinyl acetals), such as poly(vinyl butyral); polyacrylic and methacrylic esters, such as poly(methylmethacrylate), poly(n-butylmethacrylate), poly(isobutylmethacrylate), etc, polystyrene, nitrated polystyrene; polymethylstyrene, isobutylene polymers; polyesters, such as poly(ethylenealkaryloxyalkylene terephthalate), phenol-formaldehyde resins; ketone resins; polyamides, polycarbonates; polythiocarbonates, poly(ethyleneglycol-co-bishydroxyethoxyphenyl propane terephthalate); etc. Methods of making resins of this type have been described in U.S. Pat. Nos. 2,361,019 and 2,258,423. Suitable resins of the type contemplated for use in the photoconductive layers of the invention are sold under such trade names as Vitel PE-101, Cymac, Piccopale 100, Saran F-200 and Lexan 105. Other types or binders which can be used in the photoconductive layers of the invention include such materials as paraffin, mineral waxes, etc. The binders are generally used to disperse inorganic photoconductors although polymeric photoconductors could also be dispersed in the binders.

The sensitizers of this invention improve the electrical speeds of compositions containing a wide variety of photoconductors including inorganic photoconductors such as selenium, zinc oxide, titanium oxide, lead oxide, cadmium selenide, cadmium sulfide and the like and organic photoconductors including organometallic photoconductors.

Typical additional photoconductors useful herein are described below.

A. Arylamine photoconductors including substituted and unsubstituted arylamines, diarylamines, nonpolymeric triarylamines and polymeric triarylamines such as those described in U.S. Pat. Nos. 3,240,597 and 3,180,730.

B. Photoconductors represented by the formula: ##STR12## wherein Z represents a mononuclear or polynuclear divalent aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, binaphthyl, etc), or a substituted divalent aromatic radical of these types wherein said substituent can comprise a member such as an acyl group having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl, butyryl, etc), an alkyl group having from 1 to about 6 carbon atoms (e.g., methyl,ethyl, propyl, butyl, etc), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, ethoxy, propoxy, pentoxy, etc), or a nitro group; Z' represents a mononuclear or polynuclear monovalent or polynuclear monovalent aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc); or a substituted monovalent aromatic radical wherein said substituent can comprise a member such as an acyl group having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl, butyryl, etc), an alkyl group having from 1 to about 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, propoxy, pentoxy, etc), or a nitro group; Q can represent a hydrogen atom or an aromatic amino group, such as Z'NH--; b represents an integer from 1 to about 12, and L represents a hydrogen atom, a mononuclear or polynuclear aromatic radical either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc), a substituted aromatic radical wherein said substituent comprises an alkyl group, an alkoxy group, an acyl group, or a nitro group, or a poly(4'-vinylphenyl) group which is bonded to the nitrogen atom by a carbon atom of the phenyl group, these materials being more fully described in U.S. Pat. No. 3,265,496.

C. Polyarylalkane photoconductors including leuco bases of diaryl or triarylmethane dye salts, l,l,l-triaryl-alkanes wherein the alkane moiety has at least two carbon atoms and tetraarylmethanes having an amino group substituted in at least one of the aryl nuclei attached to the alkane and methane moities of the latter two classes of photoconductors which are nonleuco base materials; and also other polyarylalkanes included by the formula: ##STR13## wherein each of D, E and G is an aryl group and J is a hydrogen atom, an alkyl group, or an aryl group, at least one of D, E and G containing an amino substituent, the aryl groups attached to the central carbon atom being preferably phenyl groups, although naphthyl groups can also be used including substituted aryl groups containing substituents such as alkyl and alkoxy typically having 1 to 8 carbon atoms, hydroxy, halogen, etc, in the ortho, meta or para positions, ortho-substituted phenyl being preferred; the aryl groups can also be joined together or cyclized to form a fluorene moiety, for example; the amino substituent can be represented by the formula: ##STR14## wherein each R can be an alkyl group typically having 1 to 8 carbon atoms, a hydrogen atom, an aryl group, or together, the necessary atoms to form a heterocyclic amino group typically having 5 to 6 atoms in the ring such as morpholino, pyridyl, pyrryl, etc; at least one of D, E and G preferably being a p-dialkylaminophenyl group, when J is an alkyl group, such an alkyl group more generally has 1 to 7 carbon atoms, these materials being more fully described in U.S. Pat. No. 3,274,000, French Pat. No. 1,383,461 and in U.S. application Ser. No. 627,857, filed Apr. 3, 1967 by Seus and Goldman, now U.S. Pat. No. 3,542,544.

D. Photoconductors comprising 4-diarylamino-substituted chalcones having the formula: ##STR15## wherein :

R.sub.1 and R.sub.2 are each phenyl radicals including substituted phenyl radicals, R.sub.2 preferably having the formula: ##STR16## wherein:

R.sub.3 and R.sub.4 are each aryl radicals, aliphatic residues of 1 to 12 carbon atoms such as alkyl radicals preferably having 1 to 4 carbon atoms, or hydrogen; particularly advantageous results being obtained when R.sub.1 is a phenyl radical including a substituted phenyl radical and where R.sub.2 is diphenylaminophenyl, dimethylaminophenyl or phenyl, these materials being more fully described in Fox U.S. Pat. No. 3,526,501.

E. Non-ionic cycloheptenyl compounds which may be substituted with substituents such as (a) an aryl radical including substituted as well as unsubstituted aryl radicals, (b) a hydroxy radical, (c) a heterocyclic radical, (d) a heterocyclic radical having 5 to 6 atoms in the heterocyclic nucleus and at least one hetero nitrogen atom, and including substituted and unsubstituted heterocyclic radicals, and (e) an oxygen linked cycloheptenyl moiety. The substitution on the cycloheptenyl nucleus occurs at an unsaturated carbon atom when the cycloheptenyl moiety is a conjugated triene with no aromatic structure fused thereto. However, if there is at least one aromatic structure fused to the cycloheptenyl moiety, then the substituents are attached to a saturated carbon atom. Additional photoconductors within this class are included in one of the following formulae: ##STR17## where E.sub.2 and G.sub.2 can be either: a. a phenyl radical,

b. a naphthyl radical,

c. a heterocyclic radical having 5 to 6 atoms in the heterocyclic nucleus and at least 1 hetero nitrogen atom,

d. a hydroxyl radical, or

e. an oxygen-containing radical having a structure such that the resultant cycloheptenyl compound is a symmetrical ether;

D.sub.2 can be any of the substituents defined for E.sub.2 and .sub.2 above and is attached to a carbon atom in the cycloheptenyl nucleus having a double bond; (R.sub.9 and R.sub.10), (R.sub.11 and R.sub.12), (R.sub.5 and R.sub.6), and (R.sub.7 and R.sub.8) are together the necessary atoms to complete a benzene ring fused to the cycloheptenyl nucleus; these compounds being more fully described in U.S. application Ser. No. 654,091, filed July 18, 1967, now Pat. No. 3,533,786.

F. Compounds containing an ##STR18## nucleus including (1) unsubstituted and substituted N,N-bicarbazyls N-bicarbazyls containing substituents in either or both carbazolyl nuclei such as:

a. an alkyl radical including a substituted alkyl radical such as a haloalkyl or an alkoxy-alkyl radical,

b. a phenyl radical including a substituted phenyl radical such as a naphthyl, an aminophenyl or a hydroxyphenyl radical,

c. a halogen atom,

d. an amino radical including substituted as well as unsubstituted amino radicals such as an alkylamino or a phenylalkylamino radical,

e. an alkoxy radical,

f. a hydroxyl radical,

g. a cyano radical,

h. a heterocyclic radical such as a pyrazolyl, a carbazolyl or a pyridyl radical;

or (2) tetra-substituted hydrazines containing substituents which are substituted or unsubstituted phenyl radicals, or heterocyclic radicals having 5 to 6 atoms in the hetero nucleus, suitable results being obtained when all four substituents are not unsubstituted phenyl radicals, i.e., if at least one substituent is a substituted phenyl radical or a heterocyclic radical having 5 to 6 atoms in the hetero nucleus. Other tetra-substituted hydrazines include those having the following formula: ##STR19## wherein D.sub.1, E.sub.1, G.sub.1 and J.sub.1 are each either a. a substituted phenyl radical such as a naphthyl radical, an alkylphenyl radical, a halophenyl radical, a hydroxphenyl radical, a haloalkylphenyl radical or a hydroxyalkylphenyl radical or

b. a heterocyclic radical such as an imidazolyl radical, a furyl radical or a pyrazolyl radical. In addition, J.sub.1 and E.sub.1 can also be

c. an unsubstituted phenyl radical.

Especially preferred are those tetra-substituted hydrazines wherein both D.sub.1 and G.sub.1 are either substituted phenyl radicals or heterocyclic radicals. These compounds are more fully described in U.S. Pat. No. 3,542,546.

G. Organic compounds having a 3,3'-bisaryl-2-pyrazoline nucleus which is substituted in either five-member ring with the same or different substituents. The 1 and 5 positions on both pyrazoline rings can be substituted by an aryl moiety including unsubstituted as well as substituted aryl substituents such as alkoxyaryl, alkaryl, alkaminoaryl, carboxyaryl, hydroxyaryl and haloaryl. The 4-position can contain hydrogen or unsubstituted as well as substituted alkyl and aryl radicals such as alkoxyaryl, alkaryl, alkaminoaryl, haloaryl, hydroxyaryl, alkoxyalkyl, aminoalkyl, carboxyaryl, hydroxyalkyl and haloalkyl. Other photoconductors in this class are represented by the following structure: ##STR20## wherein:

D.sub.3, D.sub.3 ', J.sub.3 and J.sub.3 ' can be either a phenyl radical including a substituted phenyl radical such as a tolyl radical or a naphthyl radical including a substituted naphthyl radical.

E.sub.3, E.sub.3 ', G.sub.3, G.sub.3 ', L.sub.3 and L.sub.3 ' can be any of the substituents set forth above and in addition can be either a hydrogen atom or an alkyl radical containing 1 to 8 carbon atoms. These organic photoconductors are more fully described in U.S. Pat. No. 3,527,602.

H. Triarylamines in which at least one of the aryl radicals is substituted by an active hydrogen-containing group or a vinyl vinylene radical having at least one active hydrogen-containing group. These materials are more fully described in U.S. Pat. No. 3,658,520, issued Apr. 25,1972.

I. Organic-metallic compounds having at least one amino-aryl substituent attached to a Group IVa or Group Va metal atom such as silicon, germanium, tin and lead from Group IVa and phosphorus, arsenic, antimony and bismuth from Group Va. These materials can be substituted in the metallo nucleus with a wide variety of substituents but at least one of the substituents must be an amino-aryl radical. These materials are described in U.S. Pat. No. 3,647,429, issued March 7, 1972.

J. Polymeric organic photoconductors such as poly-N-vinylcarbazoles and related vinyl polymers, such materials being disclosed for example, in U.S. Pat. Nos. 3,037,861; 3,155,503; 3,418,116; 3,421,891 and 3,232,755.

K. Any other organic compound which exhibits photoconductive properties such as those set forth in Australian Pat. No. 248,402.

Representative organic photoconductors useful in this invention include the compounds listed below:

diphenylamine

dinaphthylamine

2,4,7-trinitrofluorenone

N,n'-diphenylbenzidene

N-phenyl-l-naphthylamine

N-phenyl-2-naphthylamine

N,n'-diphenyl-p-phenylenediamine

2-carboxy-5-chloro-4'-methoxydiphenylamine

p-anilinophenol

N,n'-di-2 -naphthyl-p-phenylenediamine

4,4'-benzylidene-bis-(N,N-dimethyl-m-toluidine)triphenylamine

N,N,N',N'-tetraphenyl-m-phenylenediamine

4-acetyltriphenylamine

4-hexanoyltriphenylamine

4-lauroyltriphenylamine

4-hexyltriphenylamine

4-dodecyltriphenylamine

4,4'-bis(diphenylamino)benzil

4,4'-bis(diphenylamino)benzophenone

poly[N,4"-(N,N',N'-triphenylbenzidine)]

polyadipyltriphenylamine

polysebacyltriphenylamine

polydecamethylenetriphenylamine

poly-N-(4-vinylphenyl)diphenylamine

poly-N-(vinylphenyl)-.alpha.,.alpha.'-dinaphthylamine

4,4'-benzylidene-bis(N,N-diethyl-m-toluidine)

4,4"-diamino-4-dimethylamino-2',2"-dimethyltriphenylmethane

4',4"-bis(diethylamino)-2,6-dichloro-2',2"-dimethyltriphenylmethane

4',4"-bis(diethylamino)-2',2"-dimethyldiphenyl-naphthylmethane

4',4"-bis(dimethylamino-2-chloro-2',2",4',4"-bis-(diethylamino)-2-chloro-2' ,2"-dimethyl-4-dimethylaminotriphenylmethane

4',4"-bis(diethylamino)-4-dimethylamino-2,2',2"-trimethyltriphenylmethane

4',4"-bis(dimethylamino)-2-chloro-2',2"-dimethyltriphenylmethane

4',4"-bis(dimethylamino)-2',2"-dimethyl-4-methoxytriphenylmethane

bis(4-diethylamino)-1,1,1-triphenylethane

bis(4-diethylamino)tetraphenylmethane

4',4"-bis(benzylethylamino)-2',2"-dimethyltriphenylmethane

4',4"-bis(diethylamino)-2',2"-diethoxytriphenylmethane

4,4'-bis(dimethylamino)-1,1,1-triphenylethane

1-(4-N,N-dimethylaminophenyl-1,1-diphenylethane

4-dimethylaminotetraphenylmethane

4-diethylaminotetraphenylmethane

4,4'-bis(diphenylamino)chalcone

4-diphenylamino-4'-dimethylaminochalcone

4-dimethylamino-4'-diphenylaminochalcone

4,4'-bis(dimethylamino)chalcone

4,4'-bis(diethylamino)chalcone

4-diethylamine-4'-diphenylaminochalcone

4-diphenylaminochalcone

4-dimethylaminochalcone

4'-diphenylaminochalcone

4'-dimethylaminochalcone

bis{5-(5H-dibenzo[a,d]cycloheptenyl)} ether

5-hydroxy-5H-dibenzo[a,d]cycloheptene

1-{5-(5H-dibenzo[a,d]cycloheptenyl)}-4,5-dicarbomethoxy-1,2,3-triazole

1-{5-(5H-dibenzo[a,d]cycloheptenyl)}-4,5-dibenzoyl-1,2,3-triazole

5-azido-5H-dibenzo[a,d]cycloheptene

1-{5-(10,11-dihydro-5H-dibenzo[a,d]cycloheptenyl)}-4,5-dicarbomethoxy-1,2,3 -triazole

1-{5-(10,11-dihydro-5H-dibenzo[a,d]cycloheptenyl)}-4,5-dibenzoyl-1,2,3-tria zole

4-{5-(5H-dibenzo[a,d]cycloheptenyl)}-N,N-dimethylaniline

N,N-diethyl-3-methyl-4-{5-(5H-dibenzo[a,d]cycloheptenyl)} aniline

4-{5-(5H-dibenzo[a,d]cycloheptenyl)}-1-dimethylaminonaphthalene

N,N-diethyl-3-methyl-4-{5-(10,11-dihydro-5H-dibenzo-[a,d]cycloheptenyl)}ani line

3-(4-dimethylaminophenyl)-1,3,5-cycloheptatriene

3-(4-diethylamino-2-methylphenyl)-1,3,5-cycloheptatriene

3-(4-dimethylaminonaphthyl)-1,3,5-cycloheptatriene

N,N-diethyl-3-methyl-4-{5-(5H-dibenzo[a,d]cycloheptenyl)}-aniline

tetra-.alpha.-naphthylhydrazine

tetra(3-methyl-4-hydroxyphenyl)hydrazine

tetra(m-hydroxyethylphenyl)hydrazine

tetra(2-methyl-5-chloroethylphenyl)hydrazine

tetra(2-methyl-5-hydroxyphenyl)hydrazine

tetra(1-imidazolyl)hydrazine

N,N-di-.alpha.-naphthyl-N',N',-di(3-methyl-4-hydroxyphenyl)hydrazine

N-3-furyl-N-(2-methyl-4-hydroxyphenyl)-N',N'-di-.beta.-naphthylhydrazine

tetra-.beta.-naphthylhydrazine

N,N'-di.beta.-naphthyl-N,N'-diphenylhydrazine

tetra-4-tolylhydrazine

N,N'-diphenyl-N,N'-di(3-methyl-4-hydroxyphenyl)-hydrazine

N,N'-diphenyl-N,N'-di-p-chlorophenylhydrazine phenyltri(2-methyl-5-hydroxyphenyl)hydrazine

N,N'-bicarbazyl

cyclotetrakis(3,9-carbazolylene)

6-(3-carbazolyl)-cyclotetrakis(3,9-carbazolylene)

6-(9-carbazolyl)-cyclotetrakis(3,9-carbazolylene)

3,3'-bis(3-carbazolyl-9,9'-bicarbazolyl

poly(N-vinyl-9-carbazole)

monobromo-poly(N-vinyl-9-carbazole)

3-(3-carbazolyl)-9-(9-carbazolyl)carbazole

3-(9-carbazolyl)-9,9'-bicarbazolyl

3,3'-diethyl-9,9'-bicarbazolyl

3,3'-diphenyl-9,9'-bicarbazolyl

3,3'-dichloro-9,9'-bicarbazolyl

4,4'-bis(diethylamino)-9,9'-bicarbazolyl

3,3'-diethoxy-9,9'-bicarbazolyl

1,1'-dihydroxy-9,9'-bicarbazolyl

2,2'-dicyano-9,9'-bicarbazolyltetra(p-diethylamino-phenyl)hydrazine(p-dieth ylaminophenyl)hydrazine

3,3'-bis(1,5-diphenyl-2-pyrazoline)

3,3'-bis(1-p-tolyl-5-phenyl-2-pyrazoline)

3,3'-bis(1,5-[1-naphthyl]-2-pyrazoline)

1,5-diphenyl-3-[3'-(1'-p-tolyl-5-phenyl)2'-pyrazolyl]-2-pyrazoline

3,3'-bis(1,5-diphenyl-4,5-dimethyl-2-pyrazoline)

3,3'-bis(1,4,5-triphenyl-2-pyrazoline

3,3'-bis(1,5-di-p-tolyl-4-methoxy-2-pyrazoline)

3,3'-bis(1,5-diphenyl-4-dimethylamino-2-pyrazoline)

3,3'-bis[1,5-diphenyl-4-(p-chlorophenyl)-2-pyrazoline]

3,3'-bis[1,5-diphenyl-4,5-di(p-diethylaminophenyl)-2-pyrazoline]

3,3'-bis[1,5-diphenyl-4-(p-methoxyphenyl)-5-ethyl-2-pyrazoline]

3,3'-bis(1,5-diphenyl-4-chloromethyl-2-pyrazoline)

1,5-diphenyl-4,5-dimethyl-3-[3'-(1'-p-tolyl-4'-diethyl5',5'-methylphenyl)-2 '-pyrazolyl]-2-pyrazoline

4-(p-diphenylaminophenyl)-3-buten-1-yne

p-diphenylaminostyrene

ethyl-p-diphenylaminocinnamate

methyl-p-diphenylaminocinnamate

p-diphenylaminocinnamoyl chloride

p-diphenylaminocinnamic acid N,N-diphenylamide

p-diphenylaminocinnamic acid anhydride

3-(p-diphenylaminophenyl)-2-butenoic acid

bis(p-diphenylaminobenzal)succinic acid

4-N,N-bis(p-bromophenyl)aminocinnamic acid

1-(4-diphenylamino)naphthacrylic acid

p-diphenylaminocinnamic acid

p-diphenylaminocinnamonitrile

7-diphenylamino coumarin

p-diphenylaminophenylvinylacrylic acid

p-diphenylaminobenzyl-p'-diphenylaminocinnamate

7-(p-diphenylaminostyryl)coumarin

p-diphenylaminocinnamyl alcohol

4-diphenylaminocinnamaldehyde semicarbazone

0-p-diphenylaminocinnamoyl

p'-diphenylaminobenzaldehyde oxime

p-diphenylamino cinnamaldehyde oxime

1,3-bis(p-diphenylaminophenyl)-2-propen-1-ol

methyl-p-diphenylaminobenzoate

N,N-diphenylanthranilic acid

3-p-diphenylaminophenyl-1-1-propanol

4-acetyltriphenylamine semicarbazone

ethyl-2,6-diphenyl-4-(p-diphenylaminophenyl)benzoate

1-(p-diphenylaminophenyl)-1-hydroxy-3-butyne

4-hydroxymethyltriphenylamine

1-(p-diphenylaminophenyl)ethanol

4-hydroxytriphenylamine

2-hydroxytriphenylamine

4-formyltriphenylamine oxime

4-acetyltriphenylamine oxime

1-(p-diphenylaminophenyl)hexanol

1-(p-diphenylaminophenyl)dodecanol

p-diphenylaminobenzoic acid anhydride

4-cyanotriphenylamine

p-diphenylaminobenzoic acid N,N-diphenylamide

p-diphenylaminobenzoic acid

p-diphenylaminobenzoyl chloride

3-p-diphenylaminophenylpropionic acid

4-formyltriphenylamine semicarbazone

triphenyl-p-diethylaminophenylsilane

methyl-diphenyl-p-diethylaminophenylsilane

triphenyl-p-diethylaminophenylgermane

triphenyl-p-dimethylaminophenylstannane

triphenyl-p-diethylaminophenylstannane

diphenyl-di-(p-diethylaminophenyl)stannane

triphenyl-p-diethylaminophenylplumbane

tetra-p-diethylaminophenylplumbane

phenyl-di-(p-diethylaminophenyl)phosphine

bis(p-diethylaminophenyl)phosphine oxide

tri-p-dimethylaminophenylarsine

tri-p-diethylaminophenylarsine

2-methyl-4-dimethylaminophenylarsine oxide

tri-p-diethylaminophenylbismuthine

methyl-di-(p-diethylaminophenyl)arsine

methyl-di-(p-diethylaminophenyl)phosphine

phenyl-tri-(p-diethylaminophenyl)stannane

methyl-tri-(p-diethylaminophenyl)stannane

tetra-p-diethylaminophenylgermane

diphenyl-p-diethylaminophenylsilane

p-diethylaminophenylarsine

tetrakis-[diphenyl-(p-diethylaminophenyl)plumbyl]-methane

tetrakis-[diphenyl-(p-diethylaminophenyl)stannyl]-stannane

bis[phenyl-(p-diethylaminophenyl)]dibismuthine

tri-(p-diethylaminophenyl)phosphine sulfide

di(p-diethylaminophenyl)thioxotin

4-(di-p-tolylamino)-2'-[4-(di-p-tolylamino)styryl]-stilbene

4-(di-p-tolylamino)-2',4'-dimethyl-5'-[4-(di-ptolyl)styryl]stilbene

and combinations of the above photoconductors.

In preparing the coating composition useful results are obtained where the photoconductor substance is present in an amount equal to at least about 1 weight percent of the coating composition. The upper limit in the amount of photoconductor substance present can be widely varied in accordance with usual practice. In those cases where a binder is employed, it is normally required that the photoconductor substance be present in an amount from about 1 weight percent of the coating composition to about 99 weight percent of the coating composition. A preferred weight range for the photoconductor substance in the coating composition is from about 10 weight percent to about 60 weight percent.

The amount of sensitizer that can be added to a photoconductor-incorporating layer to give effective increases in speed can vary widely. The optimum concentration in any given case will vary with the specific photoconductor and sensitizing compound used. In general, substantial speed gains can be obtained where an appropriate sensitizer is added in a concentration range from about 0.0001 to about 30 percent by weight of the film-forming coating composition. Normally, a sensitizer is added to the coating composition in an amount from about 0.005 to about 5.0 percent by weight of the total coating composition.

Various addenda that can be added to the photoconductive compositions are plasticizers, leveling agents and abrasion resistant materials such as silicones, etc.

Electrophotographic elements of the invention can be prepared with any photoconductive compound and the sensitizers of this invention in the usual manner, i.e., by blending a dispersion or solution of the photoconductive compound together with a binder, if desired, and coating or forming a self-supporting layer with the photoconductive composition. Generally, a suitable amount of the sensitizing compound is mixed with the photoconductive coating composition so that, after thorough mixing, the sensitizing compound is uniformly distributed throughout the desired layer of the coated element.

Solvents for preparing coating compositions of the present invention can include a number of solvents such as benzene, toluene, acetone, butanone, chlorinated hydrocarbons, e.g., methylene chloride, ethylene chloride, etc, ethers, e.g., tetrahydrofuran, or mixtures of these solvents, etc.

Coating thicknesses of the photoconductive composition on a support can vary widely. Normally, coating in the range of about 0.001 inch to about 0.01 inch before drying is useful for the practice of this invention. The preferred range of coating thickness is found to be in the range from about 0.002 inch to about 0.006 inch before drying although useful results can be obtained outside this range.

Suitable supporting materials for coating the photoconductive layers of the present invention can include any of a wide variety of electrically conducting supports, for example, paper (at a relative humidity about 20 percent); aluminum-paper laminates; metal foils such as aluminum foil, zinc foil, etc; metal plates, such as aluminum, copper, zinc, brass, and galvanized plates; vapor deposited metal layers such as silver, nickel or aluminum on conventional film supports such as cellulose acetate poly(ethylene terephthalate), polystyrene and the like conducting supports. An especially useful conducting support can be prepared by coating a support material such as poly(ethylene terephthalate) with a layer containing a semiconductor dispersed in a resin. Such conducting layers both with and without insulating barrier layers are described in U.S. Pat. No. 3,245,833. Likewise, suitable conducting coating can be prepared from the sodium salt of a carboxyester lactone of a maleic anhydridevinyl acetate copolymer. Such kinds of conducting layers and methods for their optimum preparation and use are disclosed in U.S. Pat. Nos. 3,007,901, 3,245,833 and 3,267,807.

The elements of the present invention can be employed in any of the well known electrophotographic processes which require photoconductive layers. One such process is the aforementioned xerographic process. As explained previously, in a process of this type the electrophotographic element is given a blanket electrostatic charge by placing the same under a corona discharge which serves to give uniform charge to the surface of the photoconductive layer. This charge is retained by the layer owing to the substantial insulating property of the layer. The electrostatic charge formed on the surface of the photoconducting layer is then selectively dissipated from the surface of the layer by exposure to light through an imagebearing transparency by a conventional exposure operation such as, for example, by contact-printing technique, or by lens projection of an image, etc, to form a latent image in the photoconducting layer. By exposure of the surface in this manner, a charge pattern is created by virtue of the fact that light causes the charge to be conducted away in proportion to the intensity of the illumination in particular area. The charge pattern remaining after exposure is then developed, i.e., rendered visible, by treatment with a medium comprising electrostatically attractable particles having optical density. The developing electrostatically attractable particles can be in the form of a dust e.g., powder, a pigment in a resinous carrier, i.e., toner, or a liquid developer may be used in which the developing particles are carried in an electrically insulating liquid carrier. Methods of development of this type are widely known and have been described in the patent literature in such patents, for example, as U.S. Pat. No. 2,297,691, and in Australian Pat. No. 212,315. In processes of electrophotographic reproduction such as in xerography, by selecting a developing particle which has as one of its components, a low-melting resin it is possible to treat the developed photoconductive material with heat and cause the powder to adhere permanently to the surface of the photoconductive layer. The heating also causes the sensitizing dye to bleach, thus rendering the background areas colorless. The heating is generally carried out in a temperature range of from about 25.degree. C to about 150.degree. C. The preferred range is from about 100.degree. C to about 135.degree. C.

The present invention is not limited to any particular mode of use of the new electrophotographic materials, and the exposure technique, the charging method, the transfer (if any) , the developing method, and the fixing method as well as the material used in these methods can be selected and adapted to the requirements of any particular technique.

Electrophotographic materials according to the present invention can be applied to reproduction techniques wherein different kinds of radiation, i.e., electromagnetic radiations as well as nuclear radiations can be used. For this reasons, it is pointed out herein that although materials according to the invention are mainly intended for use in connection with methods comprising an exposure, the term "electrophotography" wherever appearing in the description and the claims, is to be interpreted boradly and understood to comprise both xerography and xeroradiography.

The invention is further illustrated by the following examples which include preferred embodiments thereof.

EXAMPLE 1

Amorphous Diane Blue pigment having the formula: ##STR21## was crystallized by extracting the pigment in a Soxhlet extractor using distilled 1,1,2-trichloroethane. Twenty-five grams of the pigment were extracted with 3 kilos of solvent for two periods of 8 hours each. The pigment was crystalline and had an average particle size of less than one micron.

A composition containing Vitel PE 101 (poly(4,4'-isopropylidene bisphenoxyethyl-co-ethylene terephthalate) (manufactured by Goodyear Tire and Rubber Co.), 20 percent by weight of the binder of 2,4,7-trinitro-9-fluorenone (TNF) photoconductor and 1 percent by weight of binder and photoconductor of crystalline Diane Blue pigment as prepared above was coated at 0.004 inch wet thickness on a poly(ethylene terephthalate) film support which was coated with a 0.4 ND conductive nickel layer. The pigment was milled in the composition in a shaker with steel balls for a period of 1/4 to 4 hours.

The above sensitized element was compared to a control element containing the same components except that the Diane Blue pigment was amorphous.

In Examples 1-5 of the present application, Relative H & D Electrical Speeds are reported. The relative H & D electrical speeds measure the speed of a given photoconductive material relative to other materials typically within the same test group of materials. The relative speed values are not absolute speed values. However, relative speed values are related to absolute speed values. The relative electrical speed (shoulder or toe speed) is obtained simply by arbitrarily assigning a value, Ro, to one particular absolute shoulder or toe speed of one particular photoconductive material. The relative shoulder or toe speed, Rn, of any other photoconductive material, n, relative to this value, Ro, may then be calculated as follows: Rn = (A.sub.n)(.sup.Ro /.sub.Ao) wherein An is the absolute electrical speed of material n, Ro is the speed value arbitrarily assigned to the first material, and Ao is the absolute electrical speed of the first material. The absolute H & D electrical speed, either the shoulder (SH) or toe speed, of a material may be determined as follows: The material is electrostatically charged under, for example, a corona source until the surface potential, as measured by an electrometer probe, reaches some suitable initial value V.sub.o, typically about 600 volts. The charged element is then exposed to a 3000.degree.K tungsten light source through a stepped density gray scale. The exposure causes reduction of the surface potential of the element under each step of the gray scale from its initial potential V.sub.o to some lower potential V the exact value of which depends upon the amount of exposure in meter-candle-seconds received by the area. The results of these measurements are then plotted on a graph of surface potential V vs. log exposure for each step, thereby forming an electrical characteristic curve. The electrical or electrophotographic speed of the photoconductive composition can then be expressed in terms of the reciprocal of the exposure required to reduce the surface potential of any fixed selected value. The actual positive or negative shoulder speed is the numerical expression of 10.sup.4 divided by the exposure in meter-candle-seconds required to reduce the initial surface potential V.sub.o to some value equal to V.sub.o minus 100. This is referred to as the 100 volt shoulder speed. Sometimes it is desirable to determine the 50 volt shoulder speed and, in that intance, the exposure used is that required to reduce the surface potential to V.sub.o minus 50. Similarly, the actual positive or negative toe speed is the numerical expression of 10.sup.4 divided by the exposure in meter-candle-seconds required to reduce the initial potential V.sub.o to an absolute value of 100 volts. Again, if one wishes to determine the 50 volt toe speed, one merely uses the exposure required to reduce V.sub.o to an absolute value of 50 volts. An apparatus useful for determining the electrophotographic speeds of photoconductive compositions is described in Robinson et al., U.S. Pat. No. 3,449,658 issued June 10, 1969. The relative speeds obtained for the various elements tested are given in Table 1 below.

Table 1 ______________________________________ Relative Electrical H & D Speeds Example .lambda.max (sh/100 V Toe) ______________________________________ Control 635, 695 *100/3.2 *100/3.3 1 635, 695 220/16 266.7/16.7 ______________________________________ *assigned an arbitrary speed value of 100

As seen above, the speed of the element containing the crystalline pigment is unexpectedly superior to that containing the amorphous pigment.

EXAMPLE 2

This is a comparative example.

Elements containing extracted pigments not included in the instant invention were compared for speed to elements containing the same, unextracted pigments as in Example 1. Compositions containing Vitel PE 101 binder, 20 percent by weight of 4,4'-diethylamino-2,2'-dimethyltriphenylamine photoconductor and 4% by weight of both extracted and unextracted Indofast Yellow pigment having the formula: ##STR22## and 4% by weight of both extracted and unextracted Indofast Orange pigment having the formula: ##STR23## were coated and tested as in Example 1. The speeds of the elements containing the extracted pigments were substantially the same as the speeds of the elements containing the unextracted pigments. No significant speed increase was observed with the extracted pigments having formulas outside the scope of the instant invention.

EXAMPLE 3 A composition containing 1.6 g. Vitel PE 101 binder, 0.2 g. triphenylamine, 0.2 g. TNF, 0.02 g. crystalline Diane Blue pigment and 11.6 ml. dichloromethane was coated at 0.004 inch wet thickness on conductive resin-coated support. The element was tested and compared to the same element wherein the Diane Blue pigment is unextracted and amorphous. The results are shown in Table 2.

Table 2 ______________________________________ Relative Electrical H & D Speeds Example .lambda.max (sh/100 V Toe) ______________________________________ Control 635, 695 *100/11 *100/26.7 3 635, 695 117.8/15.8 333.3/38 ______________________________________ *assigned an arbitrary speed value of 100

EXAMPLE 4

A composition containing Vitel PE 101 binder, 20% by weight of 4,4'-bis(diethylamino)-2,2'-dimethyltriphenylmethane photoconductor and 4% by weight of extracted Diane Blue pigment was coated and tested as in Example 1 and compared to an element containing the same components, but with 4% by weight of amorphous Diane Blue pigment. The results are shown in Table 3.

Table 3 ______________________________________ Relative Electrical H & D Speeds Example (sh/100 V Toe) ______________________________________ Control *100/7.6 *100/6.3 4 128/10.8 105/3.6 ______________________________________ *assigned an arbitrary speed value of 100

EXAMPLE 5

A composition containing Vitel PE 101 binder, 20% by weight of triphenylamine photoconductor and 4% by weight of extracted Diane Blue pigment was coated and tested as in Example 1 and compared to an element containing the same components, but with 4% by weight of amorphous Diane Blue pigment. The results are shown in Table 4.

Table 4 ______________________________________ Relative Electrical H & D Speeds Example (sh/100 V Toe) ______________________________________ Control *100/1.5 *100/2.5 5 114.5/3.6 250/3.4 ______________________________________ *assigned an arbitrary speed value of 100

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims

1. A photoconductive composition comprising a photoconductor and a sensitizing amount of a sensitizer for the photoconductor comprising crystallized organic pigments having the formula: ##STR24## wherein n is 1 or 2; X consists of the atoms necessary to complete a naphthalene, anthracene, or ##STR25## R.sup.1,R.sup.2, and R.sup.3 are independently selected from the group consisting of hydrogen, halogen, alkoxy, NO.sub.2, alkyl, SO.sub.3 H or alkali metal salts thereof, and COOH or alkali metal salts thereof; and wherein R.sup.2 and R.sup.3 can comprise the atoms necessary to complete a phenyl, naphthyl or anthryl ring; and R is selected from the group consisting of ##STR26## or COOM wherein M is alkyl or an alkali or alkaline earth metal.

2. The photoconductive composition of claim 1 wherein the photoconductor is dispersed in an electrically insulating polymeric binder.

3. A photoconductive composition comprising a photoconductor and a sensitizing amount of a sensitizer for the photoconductor comprising crystallized organic pigments having the formula ##STR27##

4. The photoconductive composition of claim 1 wherein the pigment is crystallized by extracting the pigment in 1,1,2-trichloroethane.

5. The photoconductive composition of claim 3 wherein the pigment is crystallized by extracting the pigment in 1,1,2-trichloroethane.

6. The photoconductive composition of claim 1 wherein the photoconductor is an organic photoconductor.

7. The photoconductive composition of claim 6 wherein the organic photoconductor is selected from the group consisting of arylamines, arylalkanes, organometallics and polymeric photoconductors.

8. The photoconductive composition of claim 1 wherein the photoconductor is an inorganic photoconductor.

9. The photoconductive composition of claim 8 wherein the inorganic photoconductor is selected from the group consisting of selenium, zinc oxide, titanium oxide, cadmium sulfide, cadmium selenide and lead oxide.

10. The photoconductive composition of claim 3 wherein the photoconductor is an organic photoconductor.

11. The photoconductive composition of claim 10 wherein the organic photoconductor is selected from the group consisting of arylamines, arylalkanes, organometallics and polymeric photoconductors.

12. The photoconductive composition of claim 3 wherein the photoconductor is an inorganic photoconductor.

13. The photoconductive composition of claim 12 wherein the inorganic photoconductor is selected from the group consisting of selenium, zinc oxide, titanium oxide, cadmium sulfide, cadmium selenide and lead oxide.

14. An electrophotographic element comprising a conductive support having coated thereon a photoconductive composition comprising a sensitizing amount of sensitizer for the photoconductor comprising a crystallized organic pigment having the formula ##STR28## wherein n is 1 or 2; X consists of the atoms necessary to complete a naphthalene, anthracene, or ##STR29## R.sup.1, R.sup.2, and R.sup.3 are independently selected from the group consisting of hydrogen, halogen, alkoxy, NO.sub.2, alkyl, SO.sub.3 H or alkali metal salts thereof, and COOH or alkali metal salts thereof; and wherein R.sup.2 and R.sup.3 can comprise the atoms necessary to complete a phenyl, naphthyl or anthryl ring; and R is selected from the group consisting of ##STR30## or COOM wherein M is alkyl or an alkali or alkaline earth metal.

15. The electrophotographic element of claim 14 wherein the photoconductor is dispersed in an electrically insulating polymeric binder.

16. An electrophotographic element comprising a conductive support having coated thereon a photoconductive composition comprising a photoconductor and a sensitizing amount of a sensitizer for the photoconductor comprising a crystallized organic pigment having the formula ##STR31##

17. The electrophotographic element of claim 14 wherein the photoconductor is an inorganic photoconductor.

18. The electrophotographic element of claim 17 wherein the organic photoconductor is selected from the class consisting of arylamines, arylalkanes, organometallics and polymeric photoconductors.

19. The electrophotographic element of claim 14 wherein the photoconductor is an inorganic photoconductor.

20. The electrophotographic element of claim 19 wherein the inorganic photoconductor is selected from the class consisting of selenium, zinc oxide, titanium oxide, cadmium sulfide, cadmium selenide and lead oxide.

21. The electrophotographic element of claim 16 wherein the photoconductor is an organic photoconductor.

22. The electrophotographic element of claim 21 wherein the organic photoconductor is selected from the class consisting of arylamines, arylalkanes, organometallics and polymeric photoconductors.

23. The electrophotographic element of claim 16 wherein the photoconductor is an inorganic photoconductor.

24. The electrophotographic element of claim 23 wherein the inorganic photoconductor is selected from the class consisting of selenium, zinc oxide, titanium oxide, cadmium sulfide, cadmium selenide and lead oxide.

25. In an electrophotographic process wherein an electrostatic charge pattern is formed on an electrophotographic element the improvement characterized in that said electrophotographic element has a photoconductive layer comprising a photoconductor and a sensitizing amount of a sensitizer for the photoconductor comprising a crystallized organic pigment having the formula ##STR32## wherein n is 1 or 2; X consists of the atoms necessary to complete a naphthalene, anthracene, or ##STR33## R.sup.1, R.sup.2, and R.sup.3 are independently selected from the group consisting of hydrogen, halogen, alkoxy, NO.sub.2, alkyl, SO.sub.3 H or alkali metal salts thereof, and COOH or alkali metal salts thereof; and wherein R.sup.2 and R.sup.3 can comprise the atoms necessary to complete a phenyl, naphthyl or anthryl ring; and R is selected from the group consisting of ##STR34## or COOM wherein M is alkyl or an alkali or alkaline earth metal.

26. The electrophotographic process of claim 23 wherein the photoconductor is dispersed in an electrically insulating polymeric binder.

27. In an electrophotographic process wherein an electrostatic charge pattern is formed on an electrophotographic element the improvement characterized in that said electrophotographic element has a photoconductor layer comprising a sensitizing amount of a sensitizer for the photoconductor comprising a crystallized organic pigment having the formula: ##STR35##